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Model 46 User Guide Issue 2, September 2007
Studio Technologies, Inc. Page 23
external 2-wire power mode the Model 46
is simply an observer; whether intercom
power is present on pin 2, pin 3, or both
pins 2 and 3 is not significant.
So that the Model 46’s 2-wire-to-4-wire
interface circuits remain stable and don’t
generate audio artifacts, an auto terminate
feature is implemented. This maintains a
200 ohm termination on both pins 2 and 3
so long as neither of the 2-wire power sta-
tus LEDs is lit. To clarify, if either, or both,
the LEDs are lit then both terminations
are removed. It was felt that this method
would provide a fairly accurate indication
that an active intercom circuit is connect-
ed. In this case the 200 ohm termination is
expected to be provided elsewhere, typi-
cally as part of the external power supply,
and the Model 46’s termination is “lifted.”
Internal Power Source
When an interface’s front-panel DIP switch
is set for internal 2-wire power the Model
46’s 2-wire party-line interface supplies
30 volt DC on pin 2 of the output con-
nector. A maximum current draw of 300
milliamperes is available. This current is
sufficient to power various intercom user
devices such as small user stations and
belt packs. A common broadcast appli-
cation would be to use RTS BP325 belt
packs. Select the connected devices so
that their total current doesn’t exceed 300
milliamperes. That’s not always the easiest
figure to calculate but a web search will
generally find specifications for all com-
monly used devices. For example,
a search finds that the original version
of the BP325 consumes a maximum of
85 milliamperes of current. According to
this figure up to three of these units can
be connected to each Model 46 interface.
Newer versions of the BP325 use surface
mount component technology and have
a lower maximum current draw of 65 mil-
liamperes. Four of these “modern” BP325
units can easily be supported.
The Model 46’s intercom power supply
circuits operate under software control.
This allows detection of fault conditions
and protection of both the Model 46’s
circuitry and connected intercom user
devices. Upon initial Model 46 power
up no monitoring of the intercom power
output takes place for a period of five
seconds. This allows the Model 46’s
circuitry and the connected intercom user
devices to stabilize. The LED associated
with pin 2 will light to indicate that the
output is active. After this initial 5-second
period monitoring becomes active. A fault
condition is detected if the power on pin
2 falls below 24 volts for a continuous one
second interval. The hardware and soft-
ware responds to this condition by turning
off the power source to pin 2 and flashing
the pin 2 LED as a warning. After a five
second “cool down” interval the output re-
turns to the same condition as upon initial
power up: power is again applied to pin 2,
the pin 2 status LED will light, and moni-
toring won’t begin for another five sec-
onds. A full short-circuit condition applied
to the Model 46’s 2-wire connectors will
result in a continuous cycle of 6 seconds
on (5 seconds for start up and one second
for detection) and 5 seconds off.
As a diagnostic aid the 2-wire power
status LED associated with pin 3 remains
active in the internal power mode. When-
ever DC in excess of approximately
18 volts is present on pin 3 the LED will
light. This condition will normally never
exist but could prove useful in special
circumstances.